Economizer for steam boiler

ABSTRACT

The economizer disclosed herein includes a coil positioned in the flue gas duct of a steam boiler. The feedwater source is connected to the feedwater inlet of the boiler through the economizer coil or by-passes the economizer coil and is connected directly to the feedwater inlet by means of a three-way valve that regulates the amount of feedwater flowing through the economizer coil or by-passing the coil in response to a temperature sensing device that detects the temperature of the flue gases after passage around the coil so that the temperature of the flue gases after passing around the coil is maintained above the dew point of the flue gases.

Merritt, Jr.

Oct. 7, 1975 ECONOMIZER FOR STEAM BOILER .[75] Inventor: John H. Merritt, Jr., Atlanta, Ga. Primary Examiner K,enneth Sprague Attorney, Agent, or Firm-B. J. Powell [73] Assignee: Applied Engineering Co.,

Orangeburg, SC. [57] ABSTRACT [22] Filed: 1974 The economizer disclosed herein includes a coil posi- [21] Appl. No.: 513,555 tioned in the flue gas duct of a steam boiler. The feedwater source is connected to the feedwater inlet of the boiler through the economizer coil or by-passes the U.S. economizer Coil and is connectd i y to the feed Clwater inlet means of a three way valve that g Fleld of Search ates the amount of feedwater flowing through the 122/439 420 economizer coil or by-passing the coil in response to a temperature sensing device that detects the tempera- [56] References and ture of the flue gases after passage around the coil so UNITED STATES PATENTS that the temperature of the flue gases after passing 666,537 1/1901 Levi et a1 122/439 around the coil is maintained above the dew point of 1,565,304 l2/l925 Bell 122/421 the flue gases. 2,699,759 1/1955 Kuhner 122/421 2,704,534 3/1955 Dalin et al. 122/407 8 Clalms, 2 Drawing F lgures 1-7.0: Gas Our .P r 42 4o 44 4/ 46 28 25 LLLllllll Iii 1" 11 lIJJJl 22 20 l-liuens //v 0 fi T i 32 a! 2:, m

U.S. Patent Oct. 7,1975 3,910,236

Fun 645 Our ECONOMIZER FOR STEAM BOILER BACKGROUND OF THE INVENTION Economizers which preheat feedwater supplied to a steam boiler have been used for many years. One of the problems encountered with the use of an economizer is that corrosive condensate is formed if the temperature of the flue gases from the boiler drops below the dew point of the particular flue gases produced by the boiler. Attempts to solve this problem have been made by preheating the feedwater sufficiently high so that condensation will not occur. Examples of such attempts are illustrated in US. Pat. No. 1,612,854 which recirculates part of the feedwater back through the economizer to raise the temperature of the feedwater in the economizer and U.S. Pat. No. 2,699,759 which passes part of the feedwater through a heat exchanger in the boiler itself to preheat it prior to entry into the economizer. These systems, then, attempted to maintain any surface in the economizer with which the flue gases came into contact above the condensation temperature of the flue gases. This has served to reduce the temperature difference between the flue gases and the feedwater in the economizer which has lowered the rate of heat transferred from the flue gases and thus increased the size of the economizer coil.

Because of the difficulties associated with the prior art, the industry has simply designed economizers which are used with steam boilers that operate at a substantially constant full load rating and have not used economizers with steam boilers which operate at vary ing load ratings.

SUMMARY OF THE INVENTION These and other problems associated with the prior art are overcome by the invention disclosed herein by providing an economizer unit which allows a maximum difference in the temperature between the feedwater and the flue gases in the economizer to minimize the size of the economizer coil. Also. the invention is able to operate in conjunction with a boiler operating a varying load rating without condensate formation.

The apparatus of the invention includes an economizer coil positioned in the flue gas duct of a steam boiler. The feedwater source is connected to the feedwater inlet of the boiler through the economizer coil and directly to the feedwater inlet of the boiler bypassing the economizer coil. A three-way valve regulates the amount of feedwater flowing through the economizer coil and by-passing the coil in response to a temperature sensing device that detects the temperature of the flue gases after passage around the coil so that the temperature of the flue gases after passage around the coil is maintained above the dew point of the flue gases.

These and other features and advantages of the invention will become more clearly understood upon consideration of the following specification and accompanying drawings wherein like characters of reference designate corresponding parts through the several views and in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view illustrating the invention installed on a steam boiler; and, 7

FIG. 2 is a schematic diagram of the invention.

These figures and the following detailed description disclose specific embodiments of the invention, however, the inventive concept is not limited thereto since it may be embodied in other forms.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS Referring to FIG. 1, it will be seen that the economizer unit 10 embodying the invention is mounted on a conventional steam boiler B with a flue gas duct D and a feedwater connection FC so that the feedwater passes through a feedwater regulator valve RV into boiler B. A feedwater source FS is normally associated with boiler B to supply feedwater under pressure to the feedwater connection FC.

The economizer unit 10 includes an economizer heat exchange coil 11 best seen in FIG. 2 which is interposed in the flue gas duct D so that the flue gases flow around the coil 11 in a heat transfer relationship therewith as they pass through duct D. A feedwater flow control system 12 is also included in unit I which can selectively cause the feedwater from the feedwater source F to through the coil 1 I and then into the feedwater connection FC. and cause the feedwater to pass directly from the feedwater source FS into the feedwater connection FC by-passing the coil 11. The flow of the feedwater through the coil II and bypassing the coil II is proportioned by the flow control system so that the temperature of the flue gases after passage around coil 11 is above the dew point of the flue gases.

Previous economizers were designed based on the premise that none of the metal temperatures in the economizer could drop below the dew point to cause the gaseous corrosive compounds therein such as the sulfur oxides to form a corrosive condensate on the metal surfaces. Because the metal temperature at the feedwater inlet to the economizer is substantially that of the incoming feedwater, this necessitated the raising of the feedwater temperature entering the economizer substantially to the dew point of the flue gases. it has been found, however, that the critical temperature associated with the formation of corrosive condensates in the economizer is the average temperature of the flue gases after passage around the economizer coil. The flow control system 12 of the unit uses this concept to control the flow of feedwater through the coil 11 by controlling the average temperature of the flue gases after passage around the coil.

A fan unit 14 may be used in the duct D in conjunction with unit 10 to offset the pressure loss across the unit 10. This allows unit 10 to be introduced into duct D without affecting the operational efficiency of boiler B due to pressure loss as the flue gases pass around the coil 11.

Referring to FIG. 2, it will be seen that the coil 11 is mounted in a side wall 15 defining a passage 16 therethrough with a cross-section similar to duct D. Coil 11 has a series of heat transfer tubes 20 that extend between an inlet header 21 and an exit header 22 along a serpentine path across passage 16 through which the flue gases pass. Extended surface fins 24 may be provided on tubes 20 to increase the heat transfer efficiency thereof. The inlet header 21 is provided with a feedwater inlet 25 and the exit header 22 is provided with a feedwater outlet 26.

The feedwater inlet to economizer unit 10 is connected to the feedwater source FS through a pipe 28. The feedwater outlet 26 of economizer unit 10 is connected to one valve inlet 29 of a three-way by-pass valve 30. The valve outlet 31 of by-pass valve is connected to the feedwater connection PC on the boiler B. The other valve inlet 32 of by-pass valve 30 is also connected to the pipe 28 through a by-pass pipe 34 in parallel across coil 11 so that the by-pass valve 30 can be used to selectively control the feedwater flow through the economizer coil 11 and the by-pass pipe 34 bypassing the coil 11. These divided flows of feedwater are recombined in the by-pass valve 30 and supplied to the feedwater connection FC through outlet pipe 35 connecting the valve outlet 31 with the feedwater connection FC.

The by-pass valve 30 has a control mechanism 36 thereon which selectively operates the valve 30 so that the amount of feedwater that flows into the valve from the one valve inlet 29 or the other valve inlet 32 can be selectively controlled. While any number of by-pass valve constructions may be used, the valve 30 illustrated is a pneumatically operated proportioning valve which can proportion the flow between the valve inlets 29 and 32. One commercially available valve which operates in this manner is a commercial model three-way by-pass proportioning valve manufactured by the DeZurit Company and is pneumatically operated through a positioning cylinder actuator.

The by-pass valve 30 is operated in response to the average flue gas temperature passing out of the coil 11. This control is provided by a temperature sensor 40 positioned in the economizer unit so that the sensor 40 senses the temperature of the flue gases after passage through the coil 1 l and not the metal temperature. The temperature sensor 40 produces an electrical output which is connected to a control unit 41 that converts the electrical output of the temperature sensor into a pneumatic output that is connected to the three-way by-pass valve 30 to control the operation thereof.

The temperature sensor 40 can be any of a number of devices with the unit being illustrated as an averaging thermocouple 42. The averaging thermocouple 42 is located in passage 16 downstream of the downstream most heat transfer tube 20 of coil 11 and averages the temperature of the flue gases after passage around the coil 11 to produce an electrical output therefrom proportional to the temperature of the flue gases. While different kinds of averaging thermocouples 42 may be used, one commercially available unit which has been used successfully is a type J averaging thermocouple manufactured by Barber-Coleman Co.

The control unit 41 includes a thermocouple controller 44 designed to operate with the averaging thermocouple 42. one unit that has been used successfully is a controller manufactured by Barber-Coleman Co. as their 520 Series Setpoint Controller. The output of the thermocouple controller 44 is connected to a currentto-pneumatic transducer 45 which converts the electrical signal output from the thermocouple 42 into a pneumatic output. While any of a number of transducers 45 may be used, one unit that has operated successfully is a unit manufactured by Barber-Coleman Co, under its designation Series P02R Current-to- Pneumatic Transducer. The output of the transducer 45 is connected to the control mechanism 36 at the valve 30. Thus, it will be seen that the controller 44 can be adjusted so that the feedwater flowing through the coil 11 and by-passing the coil 11 through the by-pass pipe 34 can be proportioned by the valve 30 in response to the outlet temperature of the flue gases after passage about the coil 11 to maintain the flue gases within a prescribed temperature range. As the load on the boiler B varies, the temperature of the flue gases passing through the duct D also varies, however, the temperature sensor 40 controls the three-way valve 30 in such a way that the feedwater flow is controlled to maintain the flue gas temperature after passing through the heat exchange coil 11 at a prescribed value to prevent condensation of the corrosive chemical compounds in the flue gases on the coil 11 and the attendant corrosion of the coil 11.

The dew point of the flue gases is determined primarily by the sulfur content of the fuel being burned. Because the sulfur content in the fuel varies between the different geographical sources of the fuel, each batch of fuel burned may have a different dew point. Thus, the user adjusts the controller 44 each time the sulfur content varies to insure that the average gas temperature is above its dew point after passage about the coil 1 1.

In operation, then, it will be seen that the amount of feedwater passing through the coil 11 is directly proportional to the amount the actual temperature of the flue gases is above the prescribed temperature range set on controller 44. Likewise, the amount of feedwater by-passing coil 11 is inversely proportional to the amount the actual temperature of the flue gases is above the prescribed temperature range.

It will likewise be understood that the by-pass valve 30 can be positioned on the feedwater inlet valve of the heat exchange coil 11 without departing from the scope of the invention. A check valve may be required in the outlet of the coil 11 to prevent the feedwater on the downstream side of the coil 11 flowing back into the coil.

While specific embodiments of the invention have been disclosed herein, it will be uderstood that full use and modifications and substitutions and equivalents may be used without departing from the scope of the invention.

What is claimed as invention is:

1. An economizer unit for use with a steam boiler having a feedwater inlet, a feedwater source, and an exhaust duet through which heated flue gases are discharged, said economizer unit comprising:

an economizer heat transfer coil positioned in the duct so that the flue gases pass therearound in a heat transfer relationship therewith; and,

flow control means for selectively directing feedwater to the feedwater inlet of said boiler through said economizer coil to heat the feedwater and directly to the feedwater inlet by-passing said economizer coil, said flow control means response to the average temperature of the flue gases after passage around said economizer coil to maintain the temperature of the flue gases after passage around said economizer coil within a prescribed temperature range above the dew point of the flue gases.

2. The economizer unit of claim 1 wherein said flow control means includes:

a flow control valve operatively connecting said economizer coil and the feedwater inlet to the feedwater source, and connecting said economizer coil to the feedwater inlet to selectively direct feedwater from the feedwater source through said economizer coil to the feedwater inlet on the steam boiler and from the feedwater source to the feedwater inlet on the steam boiler by-passing said economizer coil;

temperature sensing means for sensing the temperature of the flue gases after passage around said economizer coil; and,

control circuit means operatively connected to said flow control valve and said temperature sensing means to cause said flow control valve to direct the flow of feedwater through said coil and by-passing said coil so as to maintain the flue gases in said prescribed temperature range after passage around said economizer coil.

3. The economizer unit of claim 2 wherein said tem perature sensing means is an averaging thermocouple.

4. The economizer unit of claim 3 wherein said flow control valve is a three-way proportioning valve.

5. The economizer unit of claim 4 wherein said proportioning valve includes a pneumatically operated control mechanism for operating said valve; and wherein said control circuit means includes transducer means for converting the signal generated by said thermocouple into a pneumatic output operatively connected to said control mechanism on said valve.

6. An economizer unit as set forth in claim 1 wherein said flow control means further includes a three-way valve having a first inlet, a second inlet and an outlet, said valve selectively connecting said first and second inlets to said outlet, said first inlet operatively connected to the feedwater source through said economizer coil, said second inlet connected to the feedwater source by-passing said coil. and said outlet connected to the feedwater inlet of the boiler; and temperature responsive control means operatively connected to said valve to cause the feedwater to be selectively directed through said coil and by-passing said coil to maintain the temperature of the flue gases after passage around said coil within said prescribed temperature range.

7. A method of transfering heat in the flue gases of a steam boiler to the feedwater to the boiler comprising the steps of:

positioning a heat exchange economizer coil so the flue gases from the boiler pass therearound; and,

selectively dividing the flow of feedwater to the boiler into a first flow path through the coil so that heat from the flue gases is transferred through the coil to the feedwater and into a second flow path by-passing the coil in response to the average temperature of the flue gases after passage around the coil to maintain the flue gases within an average prescribed temperature range after passage around the coil above the dew point of the flue gases.

8. The method of claim 7 wherein the step of dividing the flow of feedwater includes controlling the amount of feedwater flowing through the economizer coil directly proportionally to the amount the actual temperature of the flue gases after passage around the coil is above said prescribed temperature range, and controlling the amount of feedwater by-passing the economizer coil inversely proportionally to the amount the actual temperature of the flue gases after passage around the coil is above said prescribed temperature range. 

1. An economizer unit for use with a steam boiler having a feedwater inlet, a feedwater source, and an exhaust duct through which heated flue gases are discharged, said economizer unit comprising: an economizer heat transfer coil positioned in the duct so that the flue gases pass therearound in a heat transfer relationship therewith; and, flow control means for selectively directing feedwater to the feedwater inlet of said boiler through said economizer coil to heat the feedwater and directly to the feedwater inlet bypassing said economizer coil, said flow control means response to the average temperature of the flue gases after passage around said economizer coil to maintain the temperature of the flue gases after passage around said economizer coil within a prescribed temperature range above the dew point of the flue gases.
 2. The economizer unit of claim 1 wherein said flow control means includes: a flow control valve operatively connecting said economizer coil and the feedwater inlet to the feedwater source, and connecting said economizer coil to the feedwater inlet to selectively direct feedwater from the feedwater source through said economizer coil to the feedwater inlet on the steam boiler and from the feedwater source to the feedwater inlet on the steam boiler by-passing said economizer coil; temperature sensing means for sensing the temperature of the flue gases after passage around said economizer coil; and, control circuit means operatively connected to said flow control valve and said temperature sensing means to cause said flow control valve to direct the flow of feedwater through said coil and by-passing said coil so as to maintain the flue gases in said prescribed temperature range after passage around said economizer coil.
 3. The economizer unit of claim 2 wherein said temperature sensing means is an averaging thermocouple.
 4. The economizer unit of claim 3 wherein said flow control valve is a three-way proportioning valve.
 5. The economizer unit of claim 4 wherein said proportioning valve includes a pneumatically operated control mechanism for operating said valve; and wherein said control circuit means includes transducer means for converting the signal generated by said thermocouple into a pneumatic output operatively connected to said control mechanism on said valve.
 6. An economizer unit as set forth in claim 1 wherein said flow control means further includes a three-way valve having a first inlet, a second inlet and an outlet, said valve selectively connecting said first and second inlets to said outlet, said first inlet operatively connected to the feedwater source through said economizeR coil, said second inlet connected to the feedwater source by-passing said coil, and said outlet connected to the feedwater inlet of the boiler; and temperature responsive control means operatively connected to said valve to cause the feedwater to be selectively directed through said coil and by-passing said coil to maintain the temperature of the flue gases after passage around said coil within said prescribed temperature range.
 7. A method of transfering heat in the flue gases of a steam boiler to the feedwater to the boiler comprising the steps of: positioning a heat exchange economizer coil so the flue gases from the boiler pass therearound; and, selectively dividing the flow of feedwater to the boiler into a first flow path through the coil so that heat from the flue gases is transferred through the coil to the feedwater and into a second flow path by-passing the coil in response to the average temperature of the flue gases after passage around the coil to maintain the flue gases within an average prescribed temperature range after passage around the coil above the dew point of the flue gases.
 8. The method of claim 7 wherein the step of dividing the flow of feedwater includes controlling the amount of feedwater flowing through the economizer coil directly proportionally to the amount the actual temperature of the flue gases after passage around the coil is above said prescribed temperature range, and controlling the amount of feedwater by-passing the economizer coil inversely proportionally to the amount the actual temperature of the flue gases after passage around the coil is above said prescribed temperature range. 